Problem: XMLHttpRequest - handle server connection lost

问题

How do I handle the scenario where I making a synchronous request to the server using XMLHttpRequest and the server is not available?

xmlhttp.open("POST","Page.aspx",false);
xmlhttp.send(null);

Right now this scenario results into a JavaScript error: "The system cannot locate the resource specified"

回答1:

Ok I resolved it by using try...catch around xmlhttprequest.send

:

xmlhttp.open("POST","Page.aspx",false);              
       try
       {
       xmlhttp.send(null);
       }
       catch(e)
       {
            alert('there was a problem communicating with the server');
       }       

回答2:

Try the timeout property.

xmlHTTP.TimeOut= 2000 

回答3:

You don't check for properly returned status. By the code you gave you are doing a GET request. To properly check the status of your request, you must create an event handler for the onreadystatechange event and then inside it check if the readyState property is equal 4 and then inside the method if the status is 200.

You can find a detailed explanation here :Ajax Tutorial by Mozilla

  
xmlhttp.onreadystatechange=function()

xmlhttp.open("GET","Page.aspx",false);
{
  if (xmlhttp.readyState==4) 
  {
     if (xmlhttp.status==200)
     {
       //Ajax handling logic
     }
  }
}
xmlhttp.send(null);

来源：https://stackoverflow.com/questions/377231/problem-xmlhttprequest-handle-server-connection-lost